1. Field of the Invention
The invention relates generally to the field of amplifying devices for radio-frequency signals, and more specifically to amplifying systems for eliminating or overcoming dead spots in cellular radio-telephone systems caused by obstructions such as buildings or hills that block cellular radio-telephone signals in at least some portions of a cell.
2. Description of the Prior Art
Cellular radio-telephone systems have recently been introduced in a number of areas to provide telephone coverage for people who need to have access to telephones for communications, but who must be outside of their offices for long periods of time and are otherwise unable to get to telephones that are hard-wired to a central office over conventional telephone lines. Users of cellular systems may include, for example, people of any of a number of occupations, such as salesmen, repairmen, or pick-up or delivery men, who must do considerable amounts of automobile travel and who may not be near a telephone when their supervisors or dispatchers may need to communicate with them.
In the past, radio telephone systems have provided limited and expensive service to a number of areas. In prior systems, a radio signal from a single high-power transmitter covered an entire area. The number of subscribers who could use the system at any one time was limited by the number of channels provided for radio telephone service, which in turn was governed by the amount of radio frequency spectrum allocated to radio telephone usage in the area and the bandwidth of each channel. In most prior radio telephone systems, the number of channels, and thus the number of subscribers in a region who could use the system at any one time, was also small.
In cellular radio-telephone services, however, an area is divided into a plurality of small regions, or "cells", covered by a low-power transmitter. Currently, cellular radio telephone is provided in the 825 to 845 MHz and 870 to 890 MHz frequency bands. The higher frequency band is used for down-link transmissions, that is, transmissions from the "cell site" for reception by the subscriber. The "cell site" is the location of the transmitter, or, more specifically, the location of the antenna from which transmissions are effected for the cell. The lower frequency band is used for up-link transmissions, that is, transmissions from the subscriber for reception by the receiving equipment which is also located at the cell site.
Each of the frequency bands allocated to cellular radio telephone services in an area is divided into two parts, with one half being reserved for the local land-line telephone company and the other half being franchised to a competing service provider. Each channel has a thirty kilohertz bandwidth, allowing for 666 channels in the twenty megahertz bands, with 333 being provided to the telephone company and the same number to the franchisee.
To avoid interference between transmissions in adjacent cells, the entire twenty megahertz bandwidth is not available in all of the cells. Instead, cells are assigned certain of the channels, such that adjacent cells are not assigned the same channels. Typically, the cells may be arranged so that each cell is surrounded by six others, and so each cell may have, for example, forty eight channels provided by each of the telephone company and a franchisee (that is 333 channels divided by seven). The actual topography of the cells and number of channels in the various cells may vary depending on a number of factors. As subscribers travel between cells, the channels in which they transmit and receive the telephonic voice signals are changed in a manner and by circuitry known in the art. Thus, ninety-six simultaneous calls can take place in each cell, one over each of the channels. Using prior radio-telephone arrangements with the same bandwidth signals and bands, only ninety-six calls could take place in an entire area.
However, since the cellular radio telephone service uses relatively low power and since the wavelengths of the signals is short, obstructions such as buildings and mountains which may be present between the cell site and a subscriber at various locations in a cell, can cause significant degradation in the signal levels, in some areas reducing them to unusable levels. Increasing the power of the signals may raise them to levels which are acceptable in those areas, but that could cause several problems. First, while adjacent cells do not use the same channels, at least some of the next closest cells will use the same channels, and raising power in some cells may cause interference in those other cells. Furthermore, raising the power of a signal in one channel may cause interference between adjacent channels in adjacent cells.
In any event, increasing power of the signal transmitted from a cell site will not enhance the signal the cell site receives from the subscriber. Indeed, since the subscriber can be anywhere in the cell, even near the cell's periphery, the amount of power that a subscriber can transmit is limited at least by the criterion that the subscriber's signal also cannot interfere with signals in nearby cells. Since a subscriber, when at the cell periphery, will be closer to nearby cells than is the cell site, the limitations on signal power of a signal from a subscriber are more pronounced than on signals from the cell site.
In some circumstances, automatic gain control circuits can be used to compensate for variations in the strength of received signals, but such circuits also tend to amplify noise, which can result in a very noisy audio signal if the received signal is significantly degraded. Furthermore, if the signal is too weak to be detected, the system may determine that the call has terminated and disconnect the other party or signal an error condition. Since this may occur numerous times as a subscriber travels throughout a cell, it is desirable to enhance the signal level and also signal reception by cell equipment in areas of a cell which may otherwise be subject to obstruction.